Capture and release of acid gasses using tunable organic solvents with aminopyridine

ABSTRACT

A class of water lean, organic solvents that can bind with various acid gasses to form acid gas bound molecules having a high degree of intramolecular hydrogen bonding which enables their use as regenerable solvents for acid gas capture. Unlike the other devices described in the prior art, the present invention takes advantage of shortened distances between the portions of the molecule that form hydrogen bonds within the structures when loaded with an acid gas so as to create a molecule with a higher internal bonding affinity and a reduced proclivity for agglomeration with other molecules.

PRIORITY

This invention claims priority from and is a Continuation of U.S.application Ser. No. 16/153,104 filed Oct. 5,2018, which is a Divisionalof U.S. patent application Ser. No. 15/410,523 entitled Capture AndRelease Of Acid Gasses Using Tunable Organic Solvents With Aminopyridinefiled Jan. 19, 2017, which claims priority from provisional patentapplication no. 62/281,053 entitled System and Process for TunableOrganic Solvents for Selective Capture of CO₂ filed Jan. 20, 2016, andfrom provisional patent application No. 62/421,416 entitled Capture andRelease of Acid Gasses Using Tunable Organic Solvents with Aminopyridinefiled Nov. 14, 2016, the contents of all which are incorporated hereinby reference.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under ContractDE-AC0576RL01830 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION Field of the Invention

The invention generally relates to acid gas capture and moreparticularly to reversible acid gas capture systems and processes.

Background Information

The release of greenhouse and acid gases into the air produces local andglobal effects on the environment. The combustion of fossil fuelsgenerates acid gases such as carbon dioxide (CO₂), sulfur oxides (SO₂and COS), sulfides (H₂S) and nitrogen oxides (NOx). Fixed combustionsources, such as coal burning power plants, generate significant acidgas emissions released in their flue gas. The capture and removal of theacid gases including carbon dioxide (CO₂), sulfur oxides (SO₂, SO₃, COS,CS₂), hydrogen sulfide (H₂S), and nitrogen oxides (NOx) from flue gaswill become an even greater issue as coal becomes more prominent in theworld's future energy consumption. The capture of significant amounts ofgreenhouse and acid gases from emission sources is desired to reduce theenvironmental effects of these sources.

Current aqueous flue gas scrubbing technologies are typically too energyintensive to be used industrially or often require the use of toxicmaterials which further complicates implementation. Various currentaqueous scrubbing technologies remove sulfur oxides and nitrogen oxidesfrom flue gas, and trap these acid gases as basic salts of their acidgases (thiocarbonates, dithiocarbonates, sulfites and nitrate) using ahighly basic solution of caustic soda or lime. In these examples thebinding is stoichiometric and irreversible and results in a base thatcannot be reused.

Various binding organic liquid (BOLs) acid gas capture strategies suchas those developed by Phillip Jessop, David Heldebrant, Philip Koech andothers are described in various applications and patents such as U.S.Pat. No. 7,700,299. These technologies are a significant improvementover the prior art and have shown great promise in providing a solutionto these problems. However, in some circumstances, these binding organicliquids can be limited by problems related to an increase in viscositywith CO₂ loading which impacts the ability of these materials to bepumped and transferred from one location to another, and has createdvarious problems related to their scaling up and broader utilization.What is needed is a form of binding organic liquid that remains in aless viscous state when bound with the acid gas so as to allow pumpingand transportation of the liquid sorbents from one location to another.What is also needed is a process for tuning binding organic liquids soas to retain a desired low viscosity or other features. What is alsoneeded is a form of a binding organic liquid that retains a desiredlevel of viscosity and also allows for facile release of acid gasses andregeneration of the sorbent material. The present invention meets theseneeds.

Additional advantages and novel features of the present invention willbe set forth as follows and will be readily apparent from thedescriptions and demonstrations set forth herein. Accordingly, thefollowing descriptions of the present invention should be seen asillustrative of the invention and not as limiting in any way.

SUMMARY

The present application includes a description of a class of water lean,organic solvents that can bind with various acid gasses to formstructures having a high degree of intramolecular (internal) hydrogenbonding when bound to their acid gas which enables their use asregenerable solvents for acid gas capture. Unlike the other devicesdescribed in the prior art, the present invention takes advantage ofshortened distances between the portions of the molecule that formhydrogen bonds within the structures when loaded with an acid gas so asto create a molecule with a higher internal bonding affinity and areduced proclivity for agglomeration with other molecules.

In one set of embodiments, results of various modified CO₂ bindingorganic liquids (CO₂BOLs) are described with various designs for tuningsuch materials to effectuate acid gas capture, with charge neutrality inthe resulting bound materials so as to impart desired characteristicssuch as lowered levels of viscosity to the resulting bound materials.

In another set of embodiments, water-lean non-aqueous amine-basedsolvents that form zwitterionic carbamates for acid gas capture aredescribed. In another set of embodiments, example structures of variousof these materials including various types of amino pyridines are shownas are acid gas capture methods and processes that utilize thesematerials.

In one set of embodiments, the invention may include a method for tuningthe viscosity of a binding organic liquid comprising the step ofadjusting the structure of the acid gas capture molecule by modifyingthe internal hydrogen bonding distances within the structure to producea neutral acid gas capture material with a reduced proclivity foragglomeration with other molecules and a resulting lowered viscosity. Insome instances this distance between such hydrogen bonded portions maybe less than about 2 angstroms. In another embodiment of the inventionan acid gas capture material may have a binding organic liquid with astructure that binds to an acid gas and results in a charge neutralmolecule. In another set of embodiments a gas-selective capture sorbentfor capture and chemical binding of an acid gas is described having anamino pyridine that reversibly binds an acid gas under a first set ofconditions and releases that acid gas under a second set of conditions.The amino pyridine, or other heteroaromatic compounds such as triazines,pyrimidines, or imidazoles in such a sorbent may have any of a varietyof features including being a primary or secondary amino pyridine;having at least one R group with a carbon number selected from C1 toC18; having an R group selected from the group consisting of methyl,ethyl, propyl, iso-propyl, butyl, iso-butyl, aryl, and combinationsthereof; or having a least one R group that is an alkyl amine or adialkyl amine. In some embodiments the amino pyridine may be selectedfrom the group of 2-[(methylam ino)methyl]pyridine (2-MAMP),3-[(methylamino)methyl]pyridine (3-MAMP),4-[(methylamino)methyl]pyridine (4-MAMP), 2-[(ethylamino)methyl]pyridine (2-EAMP), 4-[(ethylamino)methyl]pyridine (4-EAMP)and 2-[(methylamino)ethyl]pyridine (2-MAEP); and derivatives andcombinations thereof. In some embodiments the capture solvent may retaina viscosity at or below about 300 cP at a maximum loading of acid gas attemperature at or below 40° C. therein.

In other embodiments, these amino pyridine solvents transform fromliquids to solid upon CO₂ capture. The resultant solid can be separatedfrom the liquids in order to intensify the CO₂-rich materials thussaving energy of regeneration by thermal heating of less material. Inone set of experiments these solid CO₂-rich materials were regeneratedat 120° C. to form the liquid sorbent.

In another set of embodiments the amino pyridine has the structure:

wherein at least one of the R groups includes an alkyl amine or adialkyl amine. In addition, in this embodiment, at least one of the Rgroups may be selected from the group consisting of Me, Et, Pr, iPr, Bu,iBu, t-Bu, cyclobutyl, cyclopentyl, and cyclohexyl. In some embodiments,R1 and R2 may each include a hetero-alkyl group selected from OMe, OEt,OPr, OBu, Ot-Bu, N(Me)₂, and N(Et)₂. In some applications the capturesolvent may comprise up to about 10% water by weight. In otherapplications the methylene repeat units (n) can be any number up to 100(n≤100).

In another application of the invention an embodiment is described whichis a method for capturing an acid gas from a stream. The method includesthe step of contacting the stream with a gas selective capture sorbenthaving an amino pyridine that reversibly binds the acid gas under afirst set of conditions to form a bound solvent, and releases the acidgas under a second set of conditions to release the bound solvent andregenerate the sorbent. In some applications, this process includesmoving the bound solvent to another location and exposing the boundsorbent to a second set of conditions. In some applications, the aminopyridine sorbent is a liquid. The acid gas may be CO₂, SO₂, COS, CS₂,H₂S, and combinations thereof. The second set of conditions may includeheating the bound solvent to a temperature between from 70° C. to 100°C., shifting polarity of the sorbent from neutral form to the non-polaramino pyridine form, or separating solid phase zwitterionic carbamatesalts or analogues thereof formed in the sorbent from a liquid phasecomprising gas-lean amino pyridines therein to at least partiallyregenerate the sorbent.

In some of these embodiments, the capture solvent may include using agas-selective capture solvent having an amino pyridine that reversiblybinds an acid gas under a first set of conditions and releases that acidgas under a second set of conditions. The amino pyridine in such asorbent may have any of a variety of features including being asecondary amino pyridine; having at least one R group with a carbonnumber selected from C1 to C18; having an R group selected from thegroup consisting of methyl, ethyl, propyl, iso-propyl, butyl, iso-butyl,aryl, and combinations thereof; or having a least one R group that is analkyl amine or a dialkyl amine. In some embodiments the amino pyridinemay be selected from the group of 2-MAMP; 3-MAMP; 4-MAMP; 2-EAMP;4-EAMP; 2-MAEP; and derivatives and combinations thereof. In someembodiments the capture solvent may retain a viscosity at or below about300 cP at a maximum loading of acid gas therein. In another set ofembodiments, the sorbents contain other heteroaromatic bases that can beused to bind with acid gasses and create similar sorbents asaminopyridines. Examples of such structures include am inopyrazines,aminopyrimidines, am inopyrazines, am inoindoles, am inoimidazoles, aminotriazoles, am inotriazines, and other similar structures. An exampleof such a structure designed for CO₂ capture has the structure:

Het=Pyridine, Pyridazine, Pyrimidine, pyrazine, indole, Imidazole,triazole and triazine wherein at least one of the R groups includes analkyl amine or a dialkyl amine. In addition, in this embodiment, atleast one of the R-groups may be selected from the group consisting ofMe, Et, Pr, iPr, Bu, iBu, t-Bu, cyclobutyl, cyclopentyl, and cyclohexyl.In some embodiments, R1 and R2 may each include a hetero-alkyl groupselected from OMe, OEt, OPr, OBu, Ot-Bu, N(Me)₂, and N(Et)₂. In someembodiments, the heteroaromatic rings consist of at least one nitrogenatom such as pyrazine, pyridazine, pyrimidine, indole, imidazole,benzimidazole, triazole and triazine. In some applications the capturesolvent may comprise up to about 10% water by weight.

Various advantages and novel features of the present invention aredescribed herein and will become further readily apparent to thoseskilled in this art from the following detailed description. In thepreceding and following descriptions I have shown and described only thepreferred embodiment of the invention, by way of illustration of thebest mode contemplated for carrying out the invention. As will berealized, the invention is capable of modification in various respectswithout departing from the invention. Accordingly, the drawings anddescription of the preferred embodiment set forth hereafter are to beregarded as illustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of one exemplary embodiment of the invention.

FIG. 2 shows an example of another embodiment of the invention

FIGS. 3A and 3B show the structures of various other embodiments of theinvention.

FIGS. 4 and 5 show the rates of CO₂ release and sorbent regeneration invarious examples of the invention

FIG. 6 shows the recyclability of one example of the present inventionover several load and unload cycles.

DETAILED DESCRIPTION OF THE INVENTION

The following description includes various embodiments of the presentinvention. It will be clear from this description of the invention thatthe invention is not limited to these illustrated embodiments but thatthe invention also includes a variety of modifications and embodimentsthereto. Therefore the present description should be seen asillustrative and not limiting. While the invention is susceptible ofvarious modifications and alternative constructions, it should beunderstood that there is no intention to limit the invention to thespecific form disclosed, but, on the contrary, the invention is to coverall modifications, alternative constructions, and equivalents fallingwithin the spirit and scope of the invention as defined in the claims.

The present detailed description includes various modified CO₂ bindingorganic liquids (CO₂BOLs) and various designs for tuning such materialsto effectuate acid gas capture while imparting desired characteristicssuch as lowered levels of viscosity to the resulting bound materials.While these embodiments are shown and described in the context of CO₂,it is to be understood that the invention is not limited thereto butincludes other acid gasses as well. In one set of embodiments,water-lean non-aqueous amine-based solvents that form zwitterioniccarbamates for acid gas capture are described. In another set ofembodiments examples structures of various materials including varioustypes of amino pyridines are shown as are acid gas capture methods andprocesses that utilize these materials.

In one typical application and usage, the present invention includes aseries of water lean or non-aqueous sorbents that are able to bind acidgasses such as CO₂, SO₂, COS, CS₂, etc. at standard temperature andpressures (STP) to form liquid carbamate salts with a generally highgravimetric capacity(˜20 wt %) but retain a desired level of viscosity.As such the bound sorbents may be pumped or transferred from onelocation to another which enables the acid gas in one location to becaptured under a first set of process conditions and then moved toanother location where the acid gas can be removed by subjecting thebound material to a second set of conditions and ideally the sorbentregenerated for later use. Stripping of these acid gasses andregeneration of the underlying sorbent can take place when conditions,such as temperature, pressure or other stimuli are altered. This enablesa cyclical process to take place wherein acid gasses are captured,bound, and then subsequently released with these binding materialsrecycled for reuse.

The solvents described in the present application have low thermalregeneration temperatures (70-100° C.) and low CO₂-rich solventviscosity compared to other water-lean materials. The regenerationtemperature can be lowered further to 60° C. by applying polarity swingassisted regeneration (PSAR) making it feasible to utilize lower gradeheat from the power plant for acid gas stripping resulting in energysavings. While others have utilized aqueous acid gas capturetechnologies made up of primary and secondary alkanol amines such asmonoethanolamine (MEA) or diethanolamine (DEA) in water for chemicalabsorption of acid gas capture materials like CO₂, high regenerationtemperatures (>120° C.), high steel corrosion due to the water load, andthermal degradation render these types of materials undesirable. Inaddition, most of these other solvents when amine-based require aco-solvent such as water or an added organic to dissolve the carbamatesalt. The present embodiments which are amine-based operate in much lessharsh (milder) conditions and do not require a co-solvent to dissolvethe CO₂ carrier to enable processing.

In one set of embodiments principles and methods for the tunability ofacid gas capture binding organic liquids for acid gas capture such asCO₂ binding in CO₂BOLs are described. A series of discoveries made atPacific Northwest National Laboratory in Richland, Wash. USA by VandaGlezakou, Roger Rousseau, and others have shown that single componentCO₂-rich CO₂BOLs do not exist entirely as zwitterionic species butrather in a dynamic equilibrium between alkyl carbonic acid (the acidform [A]) and the zwitterion form [Z]. Tests performed on thatzwitterion form revealed that various aspects of acid gas capture (andCO₂ capture specifically) by water-lean solvent systems can becontrolled by deliberate molecular modifications. Specifically, it wasfound that close proximity of amine and alcohol moieties and tunableacid/base equilibria play important roles in determining CO₂ adsorptionkinetics and bulk liquid viscosity.

The close proximity of the guanidine and alcohol moieties facilitatesthe concerted mechanism of CO₂ binding by the nucleophilic alcohol andconcurrent proton transfer to the guanidine. The overall effect is fastCO₂ binding kinetics associated with low entropic contribution to thefree energy barrier. This proximity also enables stronger internalH-bonding that favorably reduces viscosity. A high acidity at thealcohol site allows for a more efficient CO₂ activation at thetransition state and an efficient proton transfer to the amine. In someembodiments non-charged CO₂ capture solvent systems obtained byadjusting the acid/base properties of the solvent molecules, so that asignificant fraction of the CO₂-loaded molecules can exist in a partialor non-charged (acid) form may be preferable. This adjustment can beachieved by either increasing the acidity of the alcohol or bydecreasing the basicity of the amine. Experiments have shown thatnon-charged CO₂ capture systems exhibit appreciably lower viscositiesthan the analogous zwitterionic form due to decreased ionicinteractions. The guidelines outlined here for controlling CO₂ uptakekinetics and viscosity reduction can be ubiquitously applied to bothcarbonate and carbamate solvent systems.

This proximity also enables stronger internal H-bonding that favorablyaffects viscosity and assists in enabling acid gas bound moieties to bepumped from one location to another. In one set of experiments thepresence of a high acidity at the alcohol site allows for a moreefficient CO₂ activation at the transition state and an efficient protontransfer to the amine. This then leads to the concept that by adjustingthe acid/base properties of the solvent molecules so as to obtain acharge neutral molecule rather than an ionic molecule as the artcurrently describes, that CO₂ capture systems that exhibit appreciablylower viscosities than the analogous zwitterionic forms could bedesigned.

Referring first to FIG. 1, an example of one exemplary embodiment of theinvention is shown. FIG. 1 shows the methodology for the capture of anacid gas, in this case CO₂ by a binding organic liquid containingliquid. From experiments it has been shown that the acid gas bindingfree energy is one of the deciding criteria in the design of gasseparation solvents. In this particular instance, the binding organicliquid has been designed with angles that allow for a closer hydrogenbonding proximity between the positive portion of the capture moleculeand the negative portion of the bound CO₂ which results in reducing thecharge on the zwitterion moving the molecule toward a charge neutralarrangement and a resulting desired lower viscosity because theindividual molecules are structured in such a way so as to reduceagglomeration of CO₂-bound molecules. These lower viscosities in turnenable higher loadings of the bound CO₂, and flow and pumpability of thegas capture liquid to be maintained.

A series of simulations on various capture molecules (in this caseCO₂BOLs) including IPADM-2-BOL, IPADM-3-BOL, IPATBM-2-BOL, andPADM-2-BOL showed that solvated CO₂ in the vicinity of the alcohol wherethe radial rC-O distances were less than 2.00 Å binds CO₂ in the form ofan alkylcarbonate, while the H atom that originally belonged to the OHgroup remains on the guanidine N. For radial distances greater than 2.20Å, 1-IPADM-2-BOL remains in its alcohol form, and CO₂ is mostly linearwith the <OCO angle averaging ˜175°. The angle decreases to ˜165° forrC-O distances between 2.0 and 2.2 Å. CO₂ binding happens in aneffectively concerted mechanism: at rC-O ˜2.00 Å, the <OCO angledecreases to ˜165° for rC-O distances between 2.0 and 2.2 Å. CO₂ bindinghappens in an effectively concerted mechanism: at rC-O distances of˜2.00 Å, the <OCO angle becomes ˜150° with a simultaneous H transfer tothe nitrogen of the guanidine base. The CO₂ structure is consistent witha partial charge transfer to form a CO₂ (δ-) moiety and subsequentformation of a CO₃-moiety in the IPADM-2-BOL. This stronger hydrogenbonding, coupled with charge neutrality, contributes to a reduction inagglomeration in the acid gas rich environment. This reduction inagglomeration in turn results in overall reduced viscosity in thesystem. In other embodiments, viscosity is adjusted based uponmonitoring the free energy profile (i.e., ΔG ([A]-[Z]) kJ/mol) andtuning the acid-base equilibria (Keq=[A]/[Z]) between the solvated andbound CO₂ states.

The free energy profile for binding CO₂ to IPADM-2-BOL proceeds with abarrier of 16.5±1.2 kJ/mol and a binding free energy of −5.8±1.6 kJ/mol.The binding free energy is consistent with the experimentally obtainedvalues for diazabicyclo[5.4.0]-undec-7-ene (DBU) containingdual-component CO₂BOLs that range between −5.7 to -9.7 kJ/mol. Thisimplies that at 40° C., there is an equilibrium between solvated andbound CO₂. The free energy barrier of 16.5 kJ/mol and the activationenergy of 9.8 kJ/mol are compatible with the experimental observationthat this process readily occurs at 40° C.

For aqueous monoethanolamine (MEA) capture liquids, the energy barrieris more than twice that of CO₂BOLs: density functional methods give abarrier of 35.5 kJ/mol for dry MEA, and from 16 kJ/mol up to ˜63 kJ/molfor wet MEA; activation free energy estimates with the Arrheniusrelation from experimental data are ˜46.7 kJ/mol. An estimate ofactivation free energy is only ˜7 kJ/mol higher than the activationenergy, which is indicative of a small entropic contribution at thetransition state, owing to the proximity of the acid/base moieties inthe single component systems: unlike dual component systems, solventre-organization at the transition state is not required. The relativelylow barrier then suggests that capture in CO₂BOLs is likely to bediffusion limited. Because the solvent viscosity increases exponentiallywith CO₂ loading, the capture rate will decrease as more CO₂ is added.These phenomena were observed when the CO₂ absorption rates of singleand dual-component CO₂BOL solvents where measured with wetted-wallexperiments. These findings and understandings enable the design ofliquid carbamates that will have increased CO₂ uptake capability, andenable the design of carbamate salts with decreased viscosities. Bytuning the proximity of the alcohol and the amine so as to createstructures that when bound to an acid gas have a preference for theirown internal hydrogen bonding rather than agglomeration with otherstructures and creating molecules that are preferentially more chargeneutral than ionic, the problems of high viscosity in an acid gas richstate as exists in other embodiments is reduced.

Inspired by these findings, single component amines (e.g., aminopyridines) with various pendant coordinating bases were designed tocreate structures having a high degree of internal hydrogen bondingand/or acid-base equilibria favoring the non-charged acid state (e.g.,[1:1] acid [A]: zwitterion [Z] ratio) upon capture and binding of theacid gas in the bound material.

In one set of embodiments, modified CO₂ binding organic liquids withstructures favoring internal hydrogen bonded species in the boundmaterial were utilized that gave resulting viscosities that were reducedcompared to those forming primarily zwitterionic species. For example amodified 1-IPADM-2-BOL was created that formed 34% internal (neutralcharged) hydrogen bonded species and 66% external hydrogen bondedspecies that gave a resulting viscosity of ˜110 (cP) at a CO₂ loading of25 wt %.

In another set of embodiments, five modified CO₂BOLs (1-MEIPADM-2-BOL,1-IPADM-2-BOL, 1-IPADM-3-BOL, 1-IPATBM-2-BOL, PADM-2-BOL) were utilized.Table 1 lists fractions of internal hydrogen bonded species formed inthese sorbent liquids at a CO₂ loading of 25 wt %.

TABLE 1 Internal H-Bonded Viscosity Viscosity CO₂ Binding Species (MDSimulation) (Experimental) Organic Liquid (P_(int)) (cP) (cP)1-MEIPADM-2-BOL 52% 114 75 1-IPADM-2-BOL 34% 149 171 1-IPADM-3-BOL 21%190 270 1-IPATBM-2-BOL  2% 499 Very viscous 1-PADM-2-BOL <1% 950 SolidAs these data show, selecting or modifying structures of the sorbentliquid and/or tuning acid-base equilibria to obtain an increasing numberof internal hydrogen-bonded species in the bound material reducesviscosity of these CO₂ binding organic liquids.

In another set of experiments, various CO₂ binding organic liquids weremodified and tuned. The acid-base equilibria between the (solvated)organic acid and the conjugate base (i.e., bound CO₂ state) in thecapture liquid were adjusted, decreasing the free energy profile, andincreasing the number of charge neutral species that resulted indecreasing the viscosity in the resulting CO₂-bound material.

In one example, modified CO₂ binding organic liquids were created withreduced acidity of the pendant R-group of the coordinating base. AIPADM-2-BOL was modified by attaching an oxime moiety to the alcoholgroup of the coordinating base, reducing the free energy from 21.6kJ/mol to −3.1 kJ/mol and tuning the acid-base equilibrium yielding aratio of neutral-to-charged species (acid [A] : zwitterion [Z]) in thecapture liquid from 1/4000 to 3/1 in the bound material. This resultedin lower viscosity and improved capture capacity.

In another set of embodiments, modified CO₂ binding organic liquids wereutilized with structures modified to reduce basicity of the pyridinecore, for example, by attaching acidic or electronegative moieties tothe core structure. A 1-IPADM-2-BOL was modified by attaching fluorineto the pyridine core reducing the free energy in the capture liquid from21.6 kJ/mol to −5.4 kJ/mol and tuning the acid-base equilibrium yieldinga ratio of neutral-to-charged species in the capture liquid from 1/4000to 8/1 in the bound material, resulting in lower viscosity and improvedcapture capacity.

In another set of embodiments, a class of novel amino pyridine solventswas created. These non-aqueous amines demonstrated a high CO₂ capturecapacity regeneration temperatures less than 100° C., durability forabsorption and regeneration over multiple cycles without degradation,and a high water tolerance. The structures of various examples of thesematerials are shown in FIGS. 2-3.

In one set of embodiments, these amino pyridine solvents were createdthat were liquids at both CO₂ free and rich states, and werestructurally predisposed to stabilize the incipient carbamic acid uponreaction with CO₂ through formation of stable 7 and 8 membered rings,respectively, via hydrogen bonding by the acidic proton and the2-pyridine nitrogen. These solvents utilize the amine chemistry to bindCO₂ as carbamates, with internal hydrogen bonds that enable liquidproducts, thus allowing for water-free or -lean CO₂ capture liquids thatotherwise would be unachievable under standard amine compositions ofmatter. These aminopyridine materials have unprecedented high CO₂capture capacity ˜20 wt %, with low regeneration energy. These aminopyridines form a new class of materials with potential applications inacid gas capture from flue gas of coal fired power plants, separation ofacid gasses from natural gas streams and biogas, co-capture of acidgasses such as CO₂, SO₂, CO₂, CS₂ and H₂S, and other combinations ofacid gasses from natural gas and biogas streams.

In one set of experiments six of these non-aqueous amine examples(2-MAMP, 3-MAMP, 4-MAMP, 2-EAMP, 4-EAMP, 2-MAEP) were shown to captureCO₂ with high capture capacity. All of these example compounds wereliquids in the CO₂-rich state at room temperature, with a viscosity lessthan 300 cP at 40° C. Table 2 shows the CO₂ capture capacity at 25° C.of a set of six of these compounds. Table 3 shows the CO₂ capturecapacity at 40° C. These results demonstrate that the CO₂ capturecapabilities of these amine materials are approximately equivalent. Theslight increase in capture capacity at 40° C. is due to a reducedviscosity due to the rise in temperature from the exothermic reaction ofCO₂ with amines.

TABLE 2 Compound CO₂ Wt % CO₂ Mol % 2-MAMP 19.7 54.8 3-MAMP 19.5 54.14-MAMP 19.7 54.8 2-EAMP 18.2 56.5 4-EAMP 18.3 56.7 2-MAEP 17.8 55.2

TABLE 3 Compound CO₂ Wt % CO₂ Mol % 2-MAMP 21.1 58.5 3-MAMP 20.0 54.94-MAMP 18.6 51.5 2-EAMP 14.0 43.3 4-EAMP 16.7 51.8 2-MAEP 19.3 59.8

After the CO₂ is captured and held by the compounds as zwitterioniccarbamate salts under a first set of conditions, the carbamate salts canbe subjected to a second set of conditions such as higher temperatures,a reduction in pressure, mixing with other materials, or otheractivities which will cause the bound CO₂ or other acid gas to bereleased and the underlying sorbent to be regenerated. Table 4 shows theresults of a set of experiments performed on these same six compounds,wherein temperatures were raised and the percentage of CO₂ released wasmeasured. These examples show that most of these amine based sorbentmaterials can be sufficiently regenerated at temperatures of 100° C.,while 2-EAMP has a regeneration temperature of 80° C. The rates ofregeneration of these various materials at various temperatures areshown in FIGS. 4 and 5. As these figures show, the rates of regenerationare relatively fast with most of these materials releasing the bulk oftheir captured CO₂ within 5 minutes. Some materials such as 2-EAMP canbe completely stripped at a lower temperature (80° C.) in less than 10minutes. In other embodiments other materials may release at differenttemperatures including lower temperatures.

TABLE 4 Compound 70° C. 80° C. 100° 120° C. 2-MAMP 33.8 60.5 90.3 —3-MAMP 31.1 51.0 77.5 — 4-MAMP 35.1 51.3 93.8 — 2-EAMP 85 95.8 — —4-EAMP 71.4 85.1 98.8 — 2-MAEP — 34.6 76.4 90.3

In addition to acid gas release and regeneration of the solvent usingchanges in temperature, regeneration can be effectuated or supportedusing swings in pressure or polarity. In another set of experimentsembodiments of water-lean solvents were constructed that would switchpolarity upon binding with an acid gas such as CO₂. When subjected to anexternal induced polarity reduction these materials released the CO₂ andwere regenerated to their prior form and were capable of reuse for acidgas capture. This process, named polarity swing assisted regeneration(PSAR), can be coupled with other methodologies for regeneration andenables for example the regeneration of materials at a lower temperaturethan would ordinarily be required or expected. In one set of experimentsCO₂ rich 2-EAMP was able to be 81 percent stripped and regenerated at60° C. using PSAR with decane as anti-solvent.

Experiments run on these amino pyridine compounds also demonstrated thatover repeated cycles of CO₂ absorption at 25° C. and stripping andregeneration of the sorbent at 100° C. that the integrity of theunderlying amino pyridine solvent remained intact with no signs ofdegradation. Furthermore, while viscosity increased with CO₂ loading,the viscosity of these amino pyridine compounds at lower temperaturesgenerally remained at less than or equal to the viscosity of othercompounds at higher temperatures (viscosity of 2-MAEP at 40° C. isnearly the same as viscosity of IPADM-2 BOL at 75° C.). Furthermore, theaddition of up to 10 percent water to these liquids did not demonstrateany negative effect upon the CO₂ uptake and release. A mixture of CO₂rich 2-MAMP with ten percent water remained a liquid with noprecipitate, and maintained a bicarbonate-to-carbamate ratio of 5:4 per¹³C NMR analysis.

While various preferred embodiments of the invention are shown anddescribed, it is to be distinctly understood that this invention is notlimited thereto but may be variously embodied to practice within thescope of the following claims. From the foregoing description, it willbe apparent that various changes may be made without departing from thespirit and scope of the invention as defined by the following claims.

What is claimed is:
 1. A method for capturing an acid gas from a streamcontaining that acid gas the method comprising the step of; contactingthe stream with a gas selective capture sorbent having a liquid phase atless than 100 degrees C., the gas selective capture sorbent comprisingan amino pyridine that reversibly binds an acid gas under a first set ofconditions to form a solid phase zwitterionic carbamate salt andreleases the acid gas under a second set of conditions, wherein theamino pyridine has the structure:

wherein n is greater than zero and R₁ and/or R₂ includes an amineselected from the group consisting of an alkyl amine or a dialkyl amine.2. The method of claim 1 wherein one of R₁ and R₂ includes an amineselected from the group consisting of an alkyl amine or a dialkyl amine,and the other of R₁ and R₂ comprises an alkyl group slected from thegroup consisting of Me, Et, Pr, iPr, Bu, iBu, t-Bu, cyclobutyl,cyclopentyl, and cyclohexyl.
 3. The method of claim 1 wherein the gasselective capture sorbent is a lean liquid having less than 10 percentwater.
 4. The method of claim 1 wherein the gas selective capturesolvent retains a viscosity at or below about 300 cP at a maximumloading of acid gas therein.